The descriptions in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, the most commercial video compression methods and their apparatuses (H.263, H.264, MPEG-4, and H.264/AVC) use discrete cosine transform (DCT) to remove spatial redundancies in videos. At this time, transform in units of nonoverlapping blocks involves the DCT and the quantization performed independently of any correlations considered existing between adjacent blocks or pixels causing data loss and structured discontinuities of pixel values in a boundary between blocks in a low bit rate video having an especially high quantized value in a clearly visible blocking phenomenon which is called a blocking artifact. Blocking artifacts not only significantly degrade the video quality but also badly lower the video compression performance through the block distortion embedded and stored in a frame memory and the video degradation propagated to the fullest from referencing to the video in the process of motion compensation.
For this reason, a solution has been offered to use a low pass filter for smoothing boundary errors between the blocks as a way to reduce the blocking artifacts before storing a decoded image in the reference frame memory in an encoder and also to improve the image quality in a decoder and such a technique is called a deblocking filter.
However, if there exists an edge in an image at a block boundary or an onscreen object is present and the deblocking filter is applied thereto, the original image may be damaged at its component edge causing a degradation of the video quality.
In view of this, a deblocking filter in H.264/AVC video codec adaptively performs filtering at every boundary of blocks with the 4×4 pixels being the unit by which DCT and quantization are carried out. More specifically, the filtering was not adopted when a block boundary was determined as being an edge of an actual image or a boundary of an object, but the filtering was adopted when the boundary was a distortion due to the blocking artifact, in order to deal with the stated problems.
However, because the deblocking filter of H.264/AVC determines the presence of an edge only in vertical and horizontal directions, once the edge or the object boundary lies in the diagonal direction, discontinuities occur at diagonally located pixels in the boundary region between blocks leaving the concern for the blocking artifacts to occur.